WASHINGTON - Three weeks after a gas station clerk was shot and killed in Northeast D.C., police say they have cracked the case because of a red baseball cap and a trail of coins.

On the morning of Nov. 3, a man wearing a hooded jacket and ripped blue jeans made his way into a convenience store at the Valero gas station on Brentwood Road. While the clerk was in the restroom, the man made his way behind the counter and attempted to steal money from the register.

The suspect was wearing a red hat. According to D.C. police, it held the evidence they needed to make an arrest in the murder case.

Surveillance video shows the suspect grabbing the register and forcing the clerk, 68-year-old Nazir Ahmad, into a back room before shooting him twice. He also used his pistol to shoot his way out of the store – kicking out the remaining glass to get away.

However, police say the man made two critical mistakes.

He dropped coins from the register that police followed to a dumpster behind a McDonald’s restaurant located a block away. It is also where they found the man’s red baseball cap.

Police say when it was tested for DNA, the national DNA databank known as CODIS found a match for Demetrius Pierce. He is a convicted violent offender whose DNA was by law already in the system.

A few days after Ahmed was killed, we spoke with one of his co-workers.

"He was a good man. He was a man with a family,” said Mohamed Hegazy. “Imagine someone who takes someone’s life for $300 or $400. This is ridiculous. This is not right. He could have taken the money and run away without taking his life.”